WASHINGTON (CNN) --Authorities have finished draining a pond in Frederick, Maryland, and investigators are ready to search the sediment as part of the probe into the deadly 2001 anthrax attacks, city and law enforcement officials told CNN Thursday.

Frederick's police chief was informed the draining was completed late Wednesday, said Nancy Poss, the city's public information officer.

Officials estimate phase two of the operation, examining what may be left on the bottom of the pond, is expected to take several more weeks.

The draining began Monday after the FBI and the Postal Inspection Service decided to do it as part of the anthrax investigation. Investigators last year received a tip that some materials possibly used in the attacks might have been dumped into the pond.

The man-made pond, which is in a city-owned forest, is estimated to be 4 to 5 feet deep with a capacity of about 50,000 gallons. The estimated cost of draining the pond is $250,000.

The contractor hired by the FBI to drain the pond put the water into two other nearby ponds, and some of it could have overflowed into land. As part of the effort, about 100 fish also were moved to other ponds.

City officials said 10 ponds were searched last winter. Government sources told CNN several vials and a plastic box with two holes similar to something used in lab work was found in a search of one pond last December.

Anthrax-laced letters were sent in fall 2001 to the offices of U.S. Sens. Tom Daschle, D-South Dakota, and Patrick Leahy, D-Vermont, and to television network news offices in New York, and may have been sent to other places.

Five people -- two U.S. Postal Service employees in Washington, an employee at American Media Inc. in Boca Raton, Florida, a 94-year-old woman in Oxford, Connecticut, and a New York hospital supply room worker -- died of exposure to anthrax.

No suspects have been named in the anthrax investigation and no arrests have been made.

Former Army bioterrorism researcher Dr. Stephen Hatfill -- who once worked at the U.S. Army Medical Research Institute of Infectious Diseases at Fort Detrick, Maryland -- has been named a "person of interest" in the case by U.S. Attorney General John Ashcroft.

Sources told CNN last year the strain of anthrax identified in the 2001 incidents was the same as that used in experiments at USAMRIID, the Army's biological warfare defense laboratory.

Hatfill once lived in an apartment next to Fort Detrick, which is eight to 10 miles from the pond being drained. Frederick is about 50 miles northwest of Washington.

Hatfill has steadfastly denied having any role in or any knowledge of the deadly anthrax mailings. A spokesman for Hatfill, Pat Clawson, said Monday that "Steve had nothing to do with this."

"Let them go drain the ponds," Clawson told CNN. "They are not going to find anything because Steve is not involved."

Clawson called on the FBI to either charge Hatfill or clear him. He said of the cost of the draining: "If they want to [throw] away a quarter of a million dollars, be my guest."